Category Solar Cell Materials

Concluding Remarks

Gavin Conibeer1 and Arthur Willoughby2

School of Photovoltaic and Renewable Energy Engineering, University of New South Wales, Australia 2Faculty of Engineering and the Environment, University of Southampton, UK

This book aims to present the latest developments in high-efficiency photovoltaics, con­tributed by experts in the respective fields.

The physics of solar cells and of advanced concepts as presented by Jean-Francois Guille – moles, gives a very useful insight into the underlying mechanisms required for solar cells and their limiting efficiencies. The descriptions of multiple energy threshold devices and their potentials to increase efficiencies above the Schockley-Queisser limit are particularly useful...

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Quantum Antennae – Light as a Wave

The idea of a quantum antenna is to use the wave nature of light rather than its particle nature [Bailey, 1972]. Incoming light waves oscillate electrons in an antenna that has dimensions such that excited oscillations are resonant for a particular wavelength of light. Each of these oscillations is then rectified for each antennae to give a DC output. The voltage is determined by the built-in voltage of the diode (Уы = 2/3 of the bandgap of the diode semiconductor, depending on the radiative efficiency of the material). The current is determined by the number of electrons in the oscillation that are above the energy barrier determined by the diode built-in voltage...

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Nonreciprocal Devices

In Figure 9.1 one of the loss mechanisms is from radiative recombination (loss 4). In most devices this is assumed to be a minimum loss that cannot be reduced – for a cell at the radiative limit, i. e. no nonradiative recombination. This is necessary as a reciprocal device that can absorb solar wavelengths must also be able to emit those same wavelengths [Kirchoff, 1860]. However, as mentioned in Section 2.2.2 in this volume, it is possible that a nonreciprocal device could reuse some of this emitted radiation and boost efficiencies beyond the radiative limit. This is possible in theory because Kirchoff’s law applies only to time-symmetric processes. Time symmetry can be violated in a detailed process if the deeper CPT symmetry is observed...

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OTHER APPROACHES

A few other schemes have been suggested to boost photovoltaic efficiencies that do not fit into the categorisations in the preceding sections. Their physics is not yet entirely proven but they may offer intriguing possibilities for much higher efficiencies, even if only in theory at present.

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Hot-Carrier Cells

The final approach for increasing efficiencies, strategy (c) is to allow absorption of a wide range of photon energies but then to collect the photogenerated carriers before they have a chance to thermalise. A hot-carrier solar cell is just such a device that offers the possibility of very high efficiencies (the limiting efficiency is 65% for unconcentrated illumination) but with a structure that could be conceptually simple compared with other very high efficiency PV devices, such as multijunction tandem cells. (Again the physics and limiting efficiency

calculations for hot-carrier cells are outlined in Section 2.4.2...

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Thermophotonics

Thermophotonics is a variation of TPV in which the thermal source heats a luminescent diode rather than a broadband or metallic absorber as in TPV. This diode then illuminates a solar cell with a spectrum strongly peaked just above their common bandgap [Catchpole et al., 2003]. The advantage over TPV is that no additional selective emitter is required as the luminescent diode fulfills this role, (although its bandwidth is wider than an ideal selective emitter). It does, however, require a diode that has a very high luminescent radiative efficiency. Materials for such a device must be able to cool radiatively, i. e. such that they emit as much or more energy than they absorb as heat...

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Thermophotovoltaics (TPV)

A thermophotovoltaic (TPV) system consists of a narrow-bandgap photovoltaic cell (about 0.7 eV) that is illuminated by black – or greybody radiation from a hot source but at a lower temperature than the sun [Couts, 2001]. In order to give an advantage, thermal emission incident on the cell must be filtered by a selective emitter that only passes light just above the cell bandgap with photons at other energies above or below the bandgap, reflected back to the emitter. Hence, each photon’s energy or excess energy respectively, is still utilised in reheating or maintaining the temperature of the emitter. This approach would normally use waste heat from an industrial process or similar and hence not be a solar cell, but it can be coupled to an emitter heated by solar thermal energy...

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THERMAL APPROACHES

An alternative to specifically engineering multiple energy thresholds in a device or devices is to allow the photons to generate a thermal population of some sort in an absorber. The photon spectrum incident on the cell is essentially a thermal one, generated by the thermal emission from the surface of the sun at an approximate temperature of 6000 K, with an emissivity very close to that of a black body. If this energy is transferred to particles in the absorber this thermal distribution can also be transferred, with the ‘excess energy’ of the multiple energy levels of the incident photons maintained in the thermal distribution of these particles.

This thermal excess energy can be maintained in a number of different ‘particle popu­lations’: the incident photons themselves; the carriers...

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